Laser quenching method for synergistically improving obdurability and hardness of axle

Abstract

The invention discloses a laser quenching method for cooperatively improving strength, toughness and hardness of an axle, and particularly relates to the field of high-speed rail axles. The method comprises the following steps: carrying out laser quenching treatment on the axle by using a laser, and carrying out in-situ thermal cycle treatment during the laser quenching treatment. By the adoption of the technical scheme, the problem that the hardness and the ductility of an existing axle are contradictory in the laser quenching process is solved, so that the quenching microstructure is regulated and controlled through inter-pass heat circulation, and balance of the hardness and the ductility of a carbon steel part is achieved through in-situ tempering of the quenching microstructure.

Description

Technical Field

[0001] The present invention relates to the field of high-speed rail axles, and in particular to a laser quenching method for synergistically improving the strength, toughness and hardness of an axle. Background Art

[0002] The laser quenching process improves the hardness and wear resistance of carbon steel through the rapid phase transformation of martensite, and has been widely used and recognized in the field of carbon steel surface strengthening. However, the surface hardness and overall ductility of laser-quenched parts are often in conflict. For key components such as axles and gears, high requirements are placed on both surface and mechanical properties. Due to the hard and brittle nature of martensite, a single laser quenching process often fails to meet the required ductility.

[0003] Traditional tempering processes are primarily used to adjust the microstructure and mechanical properties of laser-quenched components. Due to the weak driving force for carbon diffusion, low-temperature tempering produces only a small amount of dislocation decomposition and carbon segregation, allowing the tempered components to maintain high strength and low ductility. In contrast, high-temperature tempering completely annihilates dislocations, leading to martensite recovery and recrystallization and cementite precipitation and coarsening. While softened martensite provides greater ductility for tempered components, the coarse cementite lacks the effective pinning effect to suppress grain coarsening. Combined with the annihilation of dislocations, the strength and hardness of tempered components are significantly reduced. Therefore, the technical solution of combining laser quenching with traditional tempering processes is still limited by the conflicting relationship between component hardness and ductility.

[0004] Therefore, there is an urgent need for a laser quenching method that can overcome the transmission process limitations caused by the contradictory relationship between component hardness and ductility. Summary of the Invention

[0005] The present invention aims to provide a laser quenching method for synergistically improving the strength, toughness and hardness of an axle, thereby solving the problem of the contradiction between hardness and ductility of existing axles during the laser quenching process.

[0006] In order to achieve the above-mentioned purpose, the technical solution of the present invention is as follows: a laser quenching method for synergistically improving the strength, toughness and hardness of an axle, the method is as follows: using a laser to perform laser quenching treatment on the axle, and performing in-situ thermal cycle treatment during the laser quenching treatment.

[0007] Principle and Effect of the Technical Solution: This solution utilizes the extreme non-isothermal conditions of thermal cycling to promote partial recovery and recrystallization of martensite, precipitation of fine-grained cementite, and limited decomposition of retained austenite in the component microstructure. The modulated martensite possesses submicron grains, an appropriate carbon content, and a high dislocation density, contributing to improved component strength and hardness. The softened martensite and released retained austenite enhance the component's plastic deformation capability through dislocation absorption. The fine-grained cementite suppresses microcrack propagation, improving component ductility. Thermal cycling offers opportunities for the application of laser quenching in the surface hardening of shaft components.

[0008] Furthermore, the parameters of the laser are selected as a spot diameter of 1.6 mm, a laser power of 300 W, and a scanning speed of 6 mm / s.

[0009] Through the above settings, heat input is used to heat the material to a temperature just below the melting point, ensuring that the initial ferrite and cementite are completely dissolved and completely austenitized, preparing the structure for the martensite transformation in the cooling stage, thereby achieving the quenching effect and ensuring the hardness of the quenched layer. However, it is necessary to avoid melting the material to form coarse martensite, which reduces the ductility of the material.

[0010] Furthermore, a small offset process is used during the laser quenching process.

[0011] Furthermore, the small offset is 0.3 mm.

[0012] With the above settings, the maximum thermal cycle temperature must be greater than 600°C to ensure martensite recovery and recrystallization. The number and duration of thermal cycles must ensure microstructural evolution, including cementite precipitation and austenite transformation, ultimately achieving a synergistic improvement in strength, toughness, and hardness. A minimum offset of 0.3mm reaches the critical offset value for thermal cycle microstructural control, achieving optimal processing efficiency.

[0013] Furthermore, the heating rate of the laser quenching is 1.97×10 3 ℃ / s, the cooling rate is 2.88×10 3 ℃ / s.

[0014] With the above settings, the average heating rate in the austenitizing temperature range measured by the colorimetric pyrometer can fully transform the austenite during the heating stage, but the austenite grains do not grow.

[0015] Compared with the existing technology, this solution has the following beneficial effects:

[0016] 1. This proposal provides a laser quenching method that utilizes interpass thermal cycling to synergistically enhance the hardness and ductility of the quenched layer in situ. This method significantly improves the insufficient ductility of laser-quenched components while simultaneously enhancing component strength and hardness through thermal cycling. Determination of the yield strength and tensile strength of samples laser-quenched with large offsets without thermal cycling revealed yield strengths of 787.9 MPa and 919.8 MPa, respectively, but an elongation of only 9.53%, significantly lower than that of the substrate. In contrast, samples laser-quenched with small offsets achieved yield strengths of 701.5 MPa and 865.8 MPa, respectively, representing increases of 28.19% and 21.48% over the substrate, while achieving an elongation of 16.51%, representing 94.4% of the substrate metal.

[0017] 2. Under the small offset multi-pass laser quenching process adopted in this scheme, the quenched layer will undergo complex thermal processes such as quenching and thermal cycling. During the quenching heating stage, the component is rapidly heated to a temperature just below its melting point, and the microstructure undergoes almost complete austenitization transformation. At the same time, the extremely high heating rate (1.97×10 3 ℃ / s) limits austenite grain growth, resulting in submicron grains. During the rapid cooling stage, austenite undergoes diffusionless shear transformation to martensite, and a high density of dislocations exists in the quenched martensite. Thermal cycling is essentially an extreme non-isothermal tempering process, which triggers a series of microstructural transformations in carbon steel components, including partial recovery and recrystallization of martensite, precipitation of fine-grained cementite, and decomposition of retained austenite.

[0018] 3. The present invention provides a laser quenching method that synergistically improves the strength, toughness and hardness of axles. The laser quenching layer, which is regulated by in-situ thermal cycling, achieves better strength / toughness-hardness synergy. The tempered martensite structure with fine grains, medium carbon content and dislocation density gives the laser quenched parts excellent strength and hardness, while the recrystallized martensite, fine-grained cementite and released residual austenite greatly improve the ductility of the parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Surface morphology and microhardness of samples of Example and Comparative Example;

[0020] Figure 2 : This is the microstructure diagram of the inner side and the last quenching zone of the embodiment sample and the comparative example sample;

[0021] Figure 3 1 is the XRD curve diagram of the substrate, embodiment and comparative example samples;

[0022] Figure 4 : It is the TKD microstructure characteristic diagram of the comparative sample and the example sample;

[0023] Figure 5TEM characterization of the microstructural features of the comparative sample and the example sample;

[0024] Figure 6 is the element distribution diagram of cementite in the example sample;

[0025] Figure 7 is a graph of mechanical properties of the substrate, examples, and comparative examples;

[0026] Figure 8 This is a temperature measurement curve diagram of the quenching layer in the single-pass laser quenching process of the embodiment;

[0027] Figure 9 1 is a graph showing the temperature curve, microstructure and mechanical properties of laser-treated samples at different laser powers in the embodiment;

[0028] Figure 10 This is a temperature measurement curve diagram of the quenching layer in the small offset multi-pass laser quenching process of the embodiment;

[0029] Figure 11 1 is a graph showing the mechanical properties of samples laser quenched with different offsets in the embodiment. 1 is a graph showing the mechanical properties of samples laser quenched with different offsets in the embodiment. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below through specific embodiments:

[0031] Example

[0032] like Figure 1 A laser quenching method for synergistically improving the toughness and hardness of an axle is shown. The method is as follows: the axle is laser quenched using a laser, and an in-situ thermal cycle is performed with a small offset of 0.3 mm during the laser quenching process. During the laser quenching process, the laser parameters are selected as a spot diameter of 1.6 mm, a laser power of 300 W, and a scanning speed of 6 mm / s. During the quenching heating stage, the axle is rapidly heated to 1508°C, just 32°C below its melting point, and the heating rate is 1.97×10 3 ℃ / s, the cooling rate is 2.88×10 3 ℃ / s.

[0033] Comparative Example

[0034] The only difference between this comparative example and the embodiment is that this comparative example does not adopt in-situ thermal cycle treatment, and adopts a large offset of 1.7 mm during the laser quenching process.

[0035] Laser quenching process test:

[0036] A medium-carbon steel substrate measuring φ252 × 13 mm was cut from the axle. The chemical composition of the axle is shown in Table 1 below. Laser quenching process experiments were conducted using a 5 kW continuous-wave fiber laser (IPG YLS-5000). The examples and comparative examples represent multi-pass laser quenching processes with and without thermal cycling, respectively. The resulting samples are designated LQ1 and LQ2. To fully evaluate the thermal history of the samples, the multi-pass laser quenching process was performed with more than 20 overlapping passes.

[0037] Table 1 Chemical composition of axle steel used for laser quenching (wt%)

[0038]

[0039] The macroscopic morphology of the quenched layer of the sample after the above treatment as well as the microstructure of the grains, intracrystalline dendrite structure and strengthening phase were analyzed by OM, SEM, XRD, FIB, TEM and other characterization methods combined with image processing technology. Figure 1 The surface morphology and microhardness of LQ1 and LQ2 samples are given. Figure 1 The solid boxes and dashed lines in a and c represent the sampling locations for SEM / TEM and hardness testing, respectively. The LQ1 sample has a uniform quenched layer with a thickness of approximately 0.43 μm, while the quenched layer of the LQ2 sample consists of a series of nearly independent "basin-shaped" quenched areas with only a small amount of overlap on the top of the quenched layer. Figure 1 The microhardness of the LQ1 sample shown in b is 452±28HV in the last pass, while the microhardness of the inner pass is 374±19HV, which is 17.26% lower than the last pass, but 1.84 times higher than the base metal (203HV±18HV). Figure 1 Microhardness of the LQ2 sample shown in d. The microhardness of the quenched layer varies from 456 HV to 203 HV, representing the microhardness of the laser quenched area and the base metal (BM), respectively. Figure 2 The microstructure of the inner side and the last quenching zone of LQ1 and LQ2 samples are shown. Figure 2 As shown in b, d, e, and f, the microstructures of the last sample in LQ1 and LQ2 are mainly composed of typical lath martensite with a length and width of 2 μm and 0.15 μm, respectively. Figure 2 As shown in a and c, a large number of precipitates with a grain size of less than 0.05 μm were observed inside the LQ1 quenching zone and were uniformly distributed in the martensite matrix.

[0040] Figure 3The XRD curves shown confirm the presence of some retained austenite in addition to the martensitic matrix in samples LQ1 and LQ2, indicating incomplete martensitic transformation during laser quenching and limited decomposition of retained austenite during thermal cycling. A zoomed-in view further reveals that the 2θ angles of samples LQ1 and LQ2 decrease by 0.12° and 0.15°, respectively, compared to the matrix (BM). Concomitantly, the full width at half maximum (FWHM) increases from 0.23° in the BM to 0.34° and 0.39° in samples LQ1 and LQ2, respectively. Lattice distortion in carbon steel is primarily caused by interstitial solid solution of carbon. The decrease in 2θ angles and increase in FWHM in the laser-quenched samples indicate a significant increase in the carbon content in the martensite. Furthermore, the 2θ angle of sample LQ1 increases by 0.3° and the FWHM decreases by 0.05° compared to sample LQ2, confirming partial carbon precipitation in the martensite during thermal cycling.

[0041] Analysis of microstructure transformation and mechanical properties of quenching layer:

[0042] The multi-scale microstructural transformation of the quenched layer during thermal cycling is shown in Figure 2. Figure 4-6 As shown, Figure 4 a and b are the reverse pole figures of the comparative sample and the example sample, respectively. Figure 4 c and d are the grain boundary diagrams of the comparative sample and the example sample, respectively. Figure 4 e and f are KAM diagrams of the comparative sample and the example sample, respectively. Figure 4 g and h are the recrystallized grain diagrams of the comparative sample and the example sample, respectively. Figure 4 i and j are the phase diagrams of the comparative example sample and the example sample, respectively. Figure 5 a and f are the local area diagrams of the quenching layer of the comparative sample and the embodiment sample, respectively. Figure 5 b and c are the quenched martensite diagrams of comparative samples. Figure 5 d and e are the quenched retained austenite diagrams of comparative samples. Figure 5 g is the tempered and recrystallized martensite diagram of the example sample, Figure 5 h is the cementite of the example sample, Figure 5 i and j are the tempered retained austenite of the example samples. The rapid heating and cooling of the laser quenching process makes the martensite matrix in the quenched layer significantly refined. The average grain size of the quenched martensite is only 0.36±0.18μm (as shown in Figure 2). Figure 4 a), which is reduced by 98.64% compared with the grain size of the base metal (26.5±3.8μm). At the same time, a high proportion of low-angle grain boundaries (LAGB, 59.93%) and deformed grains (9.12%) as well as a particularly high dislocation density (7.35×10 15 m -2 )( Figure 4 ac, e, g and Figure 5b) and c) show that diffusionless phase transformation leads to severe lattice distortion in quenched martensite. The strain energy stored in the lattice distortion provides additional driving force for the recovery and recrystallization of martensite during subsequent thermal cycling.

[0043] like Figure 4 As shown in bd, f, and h, since the recovery process is mainly achieved through grain boundary migration, the disappearance of straight grain boundaries, the reduction in the proportion of low-angle grain boundaries (23.71%) and deformed grains (0.54%) confirm the partial recovery of martensite during thermal cycling. In addition, the increase in the proportion of recrystallized grains (10.11%) indicates that partial recrystallization of martensite also occurred during the thermal cycling process. The average grain sizes of tempered martensite and recrystallized martensite are 0.31±0.14μm and 0.28±0.13μm, respectively. Figure 4 e, f, and Figure 5 It can be seen from cg that the dislocation density of tempered martensite is reduced to 2.11×10 15 m -2 , the dislocation density of recrystallized martensite is reduced to 0.78×10 15 m -2 The gradually decreasing dislocation density will have a significant impact on the mechanical properties of the sample.

[0044] In addition to structural transformation, carbon segregation and carbide precipitation also occur in quenched martensite during thermal cycling. The carbon content of quenched martensite is about 0.24%, while the carbon content of tempered martensite is reduced to 0.13%, and the carbon content of recrystallized martensite is reduced to 0.12%. At the same time, a large amount of cementite with a grain size of 0.035±0.028μm and an area fraction of 3.42% is discretely precipitated in the matrix ( Figure 4 j and 5h), the grain size of cementite is more than five times smaller than that of matrix (0.2 μm). In addition, the EDS image shows the distribution of elements such as C, Fe, Cr and Mn in cementite ( Figure 6 ).

[0045] from Figure 4 i, j, and Figure 5 In Figures d–i, retained austenite can be observed in both LQ1 and LQ2 samples. Granular and massive retained austenite with average particle sizes of 0.017 ± 0.0082 μm and 0.081 ± 0.026 μm were present in the LQ2 sample, while only massive retained austenite with an average particle size of 0.089 ± 0.038 μm was observed in the LQ1 sample. Therefore, the decomposition of retained austenite during thermal cycling was limited, with the area fraction decreasing from 2.01% to 1.71%. Although the massive retained austenite avoided decomposition during thermal cycling due to its larger particle size, the dislocation density was significantly reduced ( Figure 5 ej). The dislocation density of quenched retained austenite is about 6.33×10 15 m-2 , while the dislocation density of tempered retained austenite is reduced to 1.52×10 15 m -2 The massive retained austenite is greatly released during thermal cycling.

[0046] from Figure 7 As can be seen from Table 2, the LQ1 sample exhibits superior overall mechanical properties compared to both the LQ2 sample and the matrix. The yield strength and tensile strength of the LQ1 sample reach 701.5 MPa and 865.8 MPa, respectively, 28.19% and 21.48% higher than those of the matrix. The LQ1 sample also exhibits an elongation of 16.51%, representing 94.4% of that of the base material. In comparison, the yield strength and tensile strength of the LQ2 sample are 787.9 MPa and 919.8 MPa, respectively, while the elongation is only 9.53%, significantly lower than that of the matrix. A comprehensive analysis of the microhardness and mechanical properties of the samples demonstrates that components quenched by thermal cycling laser quenching can achieve higher hardness and strength, while maintaining ductility comparable to that of the matrix. The use of interpass in-situ thermal cycling successfully resolves the trade-off between hardness and ductility in the laser quenching process.

[0047] Table 2 Mechanical properties of matrix, LQ1 and LQ2 samples

[0048]

[0049] Analysis of microstructure evolution during laser quenching and thermal cycling:

[0050] The key microstructural transformations in the laser quenching process include the dissolution of carbides and ferrite during the austenitization process in the heating stage and the transformation of martensite in the cooling stage. The heating rate, peak temperature and holding time, as well as the cooling rate, determine the microstructural characteristics after the transformation. Figure 8 The temperature curve of the single laser quenching layer shown in the figure shows that during the heating stage, the substrate is rapidly heated to a peak temperature of 1508°C, which is just below the melting point (T m , 1540℃), and 697℃ higher than (811℃), the microstructure undergoes almost complete austenitization transformation. On the other hand, the extremely high heating rate (1.97×10 3 ℃ / s) and the extremely short duration (0.59s) at the austenitizing temperature restrict the growth of austenite grains, resulting in the formation of submicron grains in the quenched layer ( Figure 4 a). The calculated average cooling rate is 2.88×10 3 ℃ / s, which is much higher than the critical cooling rate of diffusion phase transformation (<5℃ / s), and austenite undergoes diffusionless shear transformation to form martensite. Due to the supersaturation of carbon content in the matrix, high shear strain is generated when austenite transforms to martensite, and thin lamellar grains are formed to minimize strain energy, forming a large dislocation density in the quenched martensite. Figure 5 d, e).

[0051] The microstructural transformation and mechanical properties of laser quenched samples are closely related to the laser quenching process parameters. Figure 9 As shown in Figure 2, when the laser power increases from the optimized 300W to 400W, the maximum temperature of the sample surface reaches 2063℃ ( Figure 9 a), the laser quenching process is transformed into the laser melting process (LM). In the subsequent cooling stage, the coarse initial austenite grains are rapidly solidified and further transformed into coarse lath martensite ( Figure 9 c). When the laser power is reduced to 200W (LQ-S), the peak temperature of the sample surface is reduced to 1362℃ and the austenitization time is reduced to 0.41s ( Figure 9 a). Due to the decrease in peak temperature and duration, cementite cannot be completely transformed during the heating stage, and the quenched layer is ultimately composed of martensite and undissolved cementite ( Figure 9 d). From the corresponding mechanical properties ( Figure 9 e) As can be seen, due to the formation of brittle plate-like martensite, the LM sample achieves a tensile strength of 1315 MPa and an elongation of only 7.21%, failing to meet the ductility requirements for the axle. In contrast, the LQ-S sample exhibits a tensile strength of only 811 MPa after quenching due to incomplete cementite dissolution and low carbon content in the martensite. At a laser power of 300 W, the quenched sample achieves a tensile strength of 919.8 MPa and an elongation of 9.53%. Further adjustments in the thermal cycle (LQ1) demonstrate that the quenched layer achieves coordinated control of strength, hardness, and ductility. Therefore, to achieve ideal laser quenching results, the quenched layer should be heated to a peak temperature close to the melting point to ensure full austenitization. At the same time, surface melting should be avoided to prevent the formation of coarse martensite.

[0052] The temperature measurement curve confirms that there is a re-quenching process in the small offset multi-pass laser quenching process, such as Figure 10 As shown, the peak temperatures of tracks 1 to 4 are all higher than The quenching and requenching process is shown. From the second to the fourth pass, the requenching process further promotes the dissolution of cementite into the initial austenite and increases the proportion of martensite. The quenching and requenching processes determine the final quenched microstructure. However, the microstructure and hardness of the last pass of the LQ1 specimen, which underwent quenching and requenching, indicate that due to the near-complete austenitization during the first quenching, the requenching has limited effect on the microstructural transformation of the quenched layer.

[0053] The peak temperature after the 5th pass is lower than (729℃) and gradually decreases, thus generating a thermal cycle to control the microstructure after quenching. The maximum peak temperature of the fifth pass reaches 685℃, and the holding time is only 0.52s, corresponding to the heating rate and cooling rate of 3.38×10 3 ℃ / s and 1.65×10 3 ℃ / s. Therefore, thermal cycling is essentially an extreme non-isothermal process, and its unique microstructural transformation includes partial recovery and recrystallization of martensite, precipitation of fine-grained cementite, and limited decomposition of retained austenite ( Figure 4-6 ). In the first stage of carbide precipitation, high heating rate and short duration reduce the annihilation of dislocations, thereby providing more nucleation sites for carbide precipitation. The presence of carbide-forming elements is another reason for the formation of fine carbides ( Figure 6 Compared with the movement of carbon in the iron lattice, the diffusion rate of substitutional alloying elements is slower and the diffusion rate in iron is several orders of magnitude lower. Since the duration is significantly shorter than the traditional tempering process, the thermal cycling process can effectively limit the spheroidization and coarsening of cementite. Therefore, the high heating rate and alloying elements of the thermal cycling process are the reasons for the precipitation of fine cementite. The difference in the distribution of retained austenite between LQ1 and LQ2 samples ( Figure 4 i and j) It can be seen that the second stage of microstructural transformation during thermal cycling is the limited decomposition of retained austenite. Granular retained austenite decomposes into ferrite and cementite, becoming another source of cementite, while the area fraction of the remaining massive retained austenite is about 1.71% ( Figure 4 j). The decomposition of retained austenite mainly depends on the diffusion of carbon in austenite, so the carbon diffusion time limits the decomposition of retained austenite. Under high temperature and short time conditions, the thermodynamic driving force for the transformation of austenite to ferrite and cementite is reduced; in addition, the increase in atomic mobility during thermal cycling reduces the dislocation density and promotes the transformation of retained austenite ( Figure 5 j).

[0054] When the temperature rises above 600℃, the martensite partially recovers and recrystallizes through dislocation movement and grain boundary migration, such as Figure 4 b, d, f, h, and Figure 5 g. The recovery kinetics of lath martensite during tempering is usually affected by two important factors: the pinning effect of carbides on grain boundary disintegration or migration and the gradual recovery of grain boundary structure over time. On the one hand, due to the strong pinning, the formation of cementite is closely related to the partial recovery of martensite grain boundaries. On the other hand, the short duration (0.23s) above 600℃ and the insufficient time for grain boundary migration also lead to the partial recovery of martensite. Figure 10It can be seen that the peak temperature exceeded 600°C only during the fifth thermal cycle. Therefore, the reason for the partial recovery of martensite during the thermal cycle is the inhibition of grain boundary migration by fine cementite and insufficient time. In addition to the recovery process, the thermal cycle also triggered the recrystallization of martensite. Since the duration of the temperature above 600°C is short and the recrystallization process requires more driving force, only partial recrystallization of martensite occurs. The area fraction of recrystallized martensite is 10.11%, with a non-equiaxed morphology and significantly reduced dislocation density.

[0055] Analysis of laser quenching process on improving mechanical properties:

[0056] From the strength point of view, the tempered martensite with a grain size reduction of 99.09% introduces grain refinement strengthening. Due to the small grain size, a small number of favorable oriented grains yield during the plastic deformation stage, resulting in a certain number of dislocations accumulating at the grain boundaries. Another strengthening factor of tempered martensite is having an appropriate carbon content. As shown in Table 3, the carbon content of tempered martensite is 0.13%, which is lower than the carbon content of quenched martensite (0.24%), but much higher than that of the ferrite matrix. The higher the carbon content, the stronger the ability to hinder dislocation movement and slip. When the screw dislocation is fixed by carbon atoms, the cross slip is restricted, and ultimately the ability of the fine structure to maintain plastic deformation by increasing dynamic dislocations will also be limited. As Figure 5 As shown in g, the dislocation density of tempered martensite is 2.11×10 15 m -2 These residual dislocations are directly related to the slip ability of screw dislocations. A higher residual dislocation density will hinder slip and plastic deformation.

[0057] Compared with tempered martensite, the carbon content (0.12%) and dislocation density (0.78×10 15 m -2 ) is lower, so the yield strength is lower ( Figure 5 g). During plastic deformation, recrystallized martensite is subjected to more local strain, and produces work hardening with tempered martensite, which is beneficial to the improvement of the overall ductility of the sample. The strength of recrystallized martensite is significantly lower than that of martensite, and there is a tendency to coarsen during conventional recrystallization. Therefore, recrystallized martensite is generally not an ideal structure for high strength performance requirements. The coarsening trend of recrystallized grains is suppressed due to the high heating rate and short duration during thermal cycling ( Figure 4 h), combined with the high strength provided by the quenched martensite, laser quenched parts have an excellent hardness / strength-ductility balance. Retained austenite is generally regarded as the main means to overcome the conflicting relationship between strength and ductility of martensitic steels, such as Figure 4As shown in j, during the thermal cycle laser quenching process, although the granular retained austenite has been decomposed, due to the rapid heating and short duration, the blocky retained austenite with a particle size of 0.089 μm is retained. In addition, the dislocation density of the retained austenite is reduced from 6.33×10 15 m -2 Reduced to 1.52×10 15 m -2 During the initial stretching phase, the retained austenite released from the small-offset laser-hardened layer can absorb more dislocations due to its lower yield strength. Consequently, the martensite remains "softened" during deformation, and its deformability improves accordingly. As the strain increases further, the retained austenite also improves ductility by alleviating stress concentration and preventing the formation of microcracks.

[0058] In the final stage of tensile deformation, microcracks will initiate and propagate in high stress concentration areas such as grain boundaries and phase boundaries. Figure 11 The mechanical properties of laser-quenched samples with offsets of 0.2mm (LQ-S1), 0.3mm (LQ1), 0.4mm (LQ-S2), 0.5mm (LQ-S3), and 1.7mm (LQ2) are presented. Compared with the optimized LQ1 process, the strength of the quenched samples gradually increased with increasing offset, while the elongation gradually decreased with the weakening of thermal cycling. The elongation of the laser-quenched samples with a smaller offset was similar to that of the LQ1 process, but the stronger thermal cycling resulted in a decrease in sample strength. Considering strength, hardness, and ductility, the LQ1 process with an offset of 0.3mm achieved the best laser quenching surface strengthening effect.

[0059] The desired surface hardness of the axle material in this example is 350-400 HV to provide sufficient wear resistance. Measurements show that the quenched layer is free of heterogeneous material interfaces, cracks, pores, and other defects, with the LQ1 process specimen achieving a microhardness of 374 HV. Furthermore, the ductility of the laser-quenched specimens is significantly improved, thanks to microstructural adjustments during the thermal cycling process. This excellent metallurgical bond and optimal balance between hardness and ductility ensure the service performance and structural safety of laser-quenched axles.

[0060] Table 3 Microstructural characteristics of laser quenching layer under different processes

[0061]

[0062]

[0063] The above are only embodiments of the present invention, and common knowledge such as the specific structure and / or characteristics of the scheme are not described in detail here. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A laser quenching method for synergistically improving the toughness and hardness of an axle, characterized in that: The method is as follows: a laser is used to perform laser quenching treatment on an axle, and an in-situ thermal cycle treatment is performed during the laser quenching treatment.

2. The laser quenching method for synergistically improving the strength, toughness and hardness of an axle according to claim 1, characterized in that: The parameters of the laser are selected as a spot diameter of 1.6 mm, a laser power of 300 W, and a scanning speed of 6 mm / s.

3. The laser quenching method for synergistically improving the toughness and hardness of an axle according to claim 1, characterized in that: A small offset treatment is used during the laser quenching process.

4. The laser quenching method for synergistically improving the strength, toughness and hardness of an axle according to claim 3, characterized in that: The small offset is 0.3 mm.

5. A laser quenching method for synergistically improving the toughness and hardness of an axle according to any one of claims 1 to 4, characterized in that: The heating rate of the laser quenching is 1.97×10 3 ℃ / s, the cooling rate is 2.88×10 3 ℃ / s.

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